1. Field of the Invention
The present invention relates to a high-strength steel pipe having a strength of API X65 grade or higher which is used for line pipes, more particularly, a high-strength steel pipe having excellent hydrogen-induced cracking resistance (HIC resistance), and a manufacturing method thereof.
2. Description of Related Arts
A steel pipe for line pipes, which is used for transportation of crude oil or natural gas containing hydrogen sulfide, is required to have what we call sour resistance including HIC resistance and stress corrosion cracking resistance (SCC resistance) as well as high strength, excellent toughness, and good weldability. It is said that HIC is caused by an internal pressure that is produced by a phenomenon that hydrogen ions created by corrosion reaction are adsorbed on the steel surface, intrude into steel as atomic hydrogen, and accumulate around nonmetallic inclusions such as MnS and hard second phases such as martensite in steel.
To prevent HIC, Unexamined Japanese Patent Publication No. 54-110119 has disclosed a manufacturing method of linepipe steels, in which by adding Ca or Ce in proper amounts relative to the amount of S, and forming fine spherical inclusions to decrease stress concentration instead of formation of needle-like MnS inclusions. Unexamined Japanese Patent Publication No. 61-60866 and Unexamined Japanese Patent Publication No. 61-165207 have disclosed a steel in which the formation of island-like martensite that functions as an origin of cracking in a center segregation region and hard phases such as martensite or bainite that function as a propagation path of cracking is restrained by a decrease in amount of segregation-prone elements (C, Mn, P, etc.), soaking treatment at a stage of slab heating, accelerated cooling during transformation at a stage of cooling, etc. Unexamined Japanese Patent Publication No. 5-9575, Unexamined Japanese Patent Publication No. 5-271766, and Unexamined Japanese Patent Publication No. 7-173536 have disclosed a steel plate having a strength of API X80 grade or higher, in which the shape of inclusions is controlled by adding Ca to a low-S steel, center segregation is restrained by lower contents of C and Mn, and high strength is provided by the addition of Cr, Mn and Ni and accelerated cooling. All of these methods for preventing HIC are methods for preventing HIC caused by center segregation.
However, a steel plate having a strength of API X65 grade or higher is usually manufactured by accelerated cooling or direct quenching, so that a near surface region of the steel plate which receives high cooling rate is more liable to be hardened than the interior thereof, and hence HIC occurs easily in the near surface region. Also, microstructure obtained by accelerated cooling consists of bainite and acicular ferrite having relatively high HIC sensitivity not only in the near surface region but also in the interior, so that the above-described method for preventing HIC caused by center segregation does not suffice. Therefore, in order to prevent HIC of steel plate completely, measures must be taken against HIC caused by the microstructure of the near surface region of steel plate and HIC caused by inclusions such as sulfide or oxide as well as HIC caused by center segregation.
On the other hand, Unexamined Japanese Patent Publication No. 7-216500 has disclosed an API X80 grade HIC-resistant steel that is composed of ferrite and bainite phases and does not contain block-like bainite or martensite phases with high HIC sensitivity. Unexamined Japanese Patent Publication No. 61-227129 and Unexamined Japanese Patent Publication No. 7-70697 have disclosed high-strength steels in which SCC resistance and HIC resistance are improved by ferritic microstructure and Mo or Ti is added to utilize precipitation strengthening by carbides.
However, the microstructure of the high-strength steel described in Unexamined Japanese Patent Publication No. 7-216500 consists of bainite phases with relatively high HIC sensitivity. Also, this steel is high in manufacturing cost because the content of S and Mn is restricted severely and Ca treatment is necessary. The microstructure of the high-strength steels described in Unexamined Japanese Patent Publication No. 61-227129 and Unexamined Japanese Patent Publication No. 7-70697 consists of ductile ferritic phases, so that the HIC sensitivity is very low, while the strength is low. In order to obtain higher strength for the steel described in Unexamined Japanese Patent Publication No. 61-227129, large amounts of C and Mo are added, cold-rolling is performed after quench-and-temper, and tempering is performed again to precipitate a large amount of carbides, resulting in increased manufacturing cost. The steel described in Unexamined Japanese Patent Publication No. 7-70697 cannot achieve high strength stably because Ti is added to obtain high strength by utilizing precipitation strengthening of TiC at a stage of coiling, but TiC is liable to be coarsened by the influence of coiling temperature. Although high strength can be achieved stably by adding large amount of Ti, the toughness of heat-affected zone deteriorates significantly when the welding such as electric resistance welding or submerged arc welding are applied.